Stars known as blue stragglers have pretty much discovered the cosmic equivalent of the fountain of youth - they should be old and nearly out of fuel, yet they continue to burn hot and bright blue like a young star.

Figuring out just what makes these blue stragglers tick has long baffled astronomers, but now researchers at Northwestern and the University of Wisconsin think they have the answer. It's all about a process known as mass transfer, which is a nicely scientific way of describing what these blue stragglers are really all about - they steal huge amounts of mass from a giant companion star, which gives them the extra fuel they need to keep burning longer. Their poor companion is ultimately stripped bare, leaving nothing behind but a white dwarf core.

The researchers hit upon this explanation when they realized that the vast, vast majority of blue stragglers are in binary star systems. That means they have a ready supply of extra fuel just waiting to be gobbled up. The fact that most of the blue stragglers' companions are now white dwarfs serves as indirect evidence - they're the desiccated remains of once giant stars that lost their mass to the blue stragglers.

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Northwestern astronomer Aaron M. Geller explains:

"It's really the companion star that helped us determine where the blue straggler comes from. The companion stars orbit at periods of about 1,000 days, and we have evidence that the companions are white dwarfs. Both point directly to an origin from mass transfer. We think we have a good understanding of stellar evolution, but it doesn't predict blue stragglers. People have been trying to explain the origin of blue stragglers since their discovery in 1953, and now we have the detailed observations needed to identify how they were created. I've always enjoyed trying to get to the bottom of a mystery."